In December 2008, Demi Lovato appeared on the red carpet at Miley Cyrus' Disney-sponsored 16th birthday bash with several red marks on her wrist. Armchair psychologists were quick to diagnose her as a cutter, a charge her rep adamantly denied, insisting the wounds were the result of too-tight "gummy bracelets."

But after checking out of rehab in January following three months of treatment for cutting, bulimia and other emotional issues, Lovato, 18, is now talking openly about her problems with self-mutilation, which she says began when she was just 11.

"It was a way of expressing my own shame, of myself, on my own body," she reveals to Robin Roberts in an interview airing Friday on "20/20." "I was matching the inside to the outside. And there were some times where my emotions were just so built up, I didn't know what to do. The only way that I could get instant gratification was through an immediate release on myself."

Demi also admits that she's had a "really unhealthy relationship with food" for the last decade, starting with compulsive eating when she was 8 and seguing into bulimia and severe eating restrictions, problems she blames on being bullied.

"I literally didn't know why they were being so mean to me. And when I would ask them why, they would just say, 'Well, you're fat,'" she recalls. "I developed an eating disorder, and that's kind of what I've been dealing with ever since."

Lovato's troubles came to a head last October while on tour in South America with the Jonas Brothers. After an apparent night of partying, she allegedly punched a female backup dancer. She reached an undisclosed financial settlement with the dancer in December.

"I was performing concerts on an empty stomach," Demi explains. "I was losing my voice from purging. I was self-medicating. I was not taking medication for depression, and I literally was so emotionally whacked out that I took it out on someone that meant a lot to me."

Declares the starlet, "I take 100 percent, full responsibility. I feel horrible. [She] was my friend."

Lovato made a beeline for rehab following an intervention with her family and management team: "They sat me down and said, 'You can't live like this.'"

While in treatment, she kept a picture of her little sister nearby to remind her that she needed to set an example, and now she wants to educate others about what she went through.

"The real reason why I'm sitting down with you, is to open up the eyes of so many young girls," says Demi, "that it doesn't have to be this way."

Another step Lovato says she's taking on the road to recovery: leaving her Disney Channel show "Sonny With a Chance."

"It made sense for me to go ahead and leave the show to focus on my music," she tells People. "It's kind of sad for me that a chapter of my life has ended but there couldn't be a better time for me to move on."

According to Demi, "I don't think going back to 'Sonny' would be healthy for my recovery. Being in front of a camera would make me nervous."

Responded the Disney Channel, "We respect Demi's decision to focus full time on her music and not immediately return to her acting career. She is a talented young woman and our hearts are with her as she continues to take action to improve her health and bounce back from adversity."

The show will continue under a new format and name -- "So Random!" -- in June.